It is well known that a non-volatile memory is still able to maintain the stored data even after its power is turned off. Recently, mask read-only memory (Mask ROMs) have been developed as a type of non-volatile memory. When Mask ROMs are formed, the data has been stored. A user can only use/read the data stored in the Mask ROMs; and is unable to program the data. Thus, the Mask ROMs have a relatively high dependability. Further, the Mask ROMs are formed by using masks. Thus, the fabrication process of the Mask ROMs is relatively simple, and the production cost is relatively low. Therefore, Mask ROMs have been widely used in a variety of electronic products.
Currently, MOS transistors are used as the memory cells of Mask ROMs. That is, the channel regions of the MOS transistors are doped by ion implantation or not doped by ion implantation to obtain different on-state voltages of the MOS transistors. The different on-state voltages are used to store the “0” or “1” of the data. However, the area of the MOS transistors are relatively large, and the memory density of the Mask ROMs is relatively small. With the continuous shrinking of the critical dimension of MOS transistors, the MOS transistors may have short-channel effect, hot carrier effect, and punch-through effect, etc.
To overcome such issues, diodes have been used to substitute the MOS transistors as the memory cells in the Mask ROMs. Although, diodes, i.e., PN junctions, occupy less area of the Mask ROMs, the on-state voltage of the diodes is relatively large. For example, the on-stage voltage of the diodes is in a range of approximately 0.7V˜1V. Thus, the power consumption of data reading process is relatively large. Further, the PN junctions are turned on by the minority carriers. Thus, the Mask ROMs using the PN junction diodes as the memory cells have a relatively low running speed during the data reading process.
The disclosed device structures and methods are directed to solve one or more problems set forth above and other problems.